The present invention relates generally to data communication networks, and specifically to an improved local area network bridge unit for transferring data communications originating on a first network to a second network.
In local area data communications network topologies, it is often necessary or advantageous to couple two separate communications networks, each having a plurality of dataframe originating and receiving stations, to one another by a network bridge unit. In the most simplistic design the network bridge unit operates to copy all dataframes originating on one network to the other. Thus, each network sees all dataframes that are transmitted on either network.
However, as the number of stations on one or both of the networks increase, the dataframe traffic can overload such a simplistic design and, therefore, it is often necessary to provide the network bridge unit with circuitry to monitor the dataframe traffic on either of the networks, transporting only those dataframes from one network specifically designated for a receiving station located on the other network. Thus, microprocessor-based network bridge units are used to provide "filtering," in the sense that dataframes communicated on a network bound for stations on that same network are "filtered" from those that are to be transported to the other network.
The filter operation depends upon use of identification (ID) information contained in the dataframes transmitted. Common dataframe protocol specifies that the dataframe contain the ID of the station or node from which the dataframe originated, as well as the ID of the destination node. In this way, the network bridge unit can receive the dataframe, extract the destination ID, and determine therefrom whether the dataframe is to be transported from the network on which it was first communicated to the other network.
As available technology increases the available bandwidth on networks, more nodes can be connected or coupled to a network, significantly increasing the traffic, and the rate of traffic. Thus, for example, in FDDI (ANSI X3T9.5) local area networks, 100 megabit data rates are not uncommon. In order to meet the necessary timing requirements for small dataframes, a network bridge unit is severely put upon to perform the necessary comparison in a very small amount of time. This places a severe requirement upon microprocessor-based network bridge units, requiring either expensive, high performance microprocessors, or some other approach.